Major Sedimen\ Types 77
various processes, including recrystallization (crystal growth within pre-existing
solids), can be reconstructed by studying the distribution of the elements and compounds involved, within the solids and the interstitial waters. The distribution of
certain isotopes (oxygen, carbon, strontium) is of special interest in this context,
because dissolution, migration, and reprecipitation under various conditions result in
altered ratios of the isotopes.
Of course, diagenetic processes are extremely important for hydrocarbon source
rocks, migration and reservoir rock porosities, and permeabilities (Fig. 10.2).
3.3.3 Residence Time. For steady-state conditions, output must equal input. Thus the
seawater has to rid itself of all new salts coming in, in the same proportions as they
are added. Where are the "sinks" for this material? The quantitative assessment of
sinks is a major geochemical problem. For calcium carbonate the sink is calcareous
skeletons built by organisms; for silica it is opaline skeletons. The metals presumably
leave the ocean in newly formed minerals such as authigenic clay, oxides, and sulfides, and in zeolites, as well as in alteration products resulting from reactions between hot basalt and seawater, at the ridge crest. The sulfur is precipitated in heavy
metal sulfides in anaerobic sediments near the land. Some salt leaves with the pore
waters in the sediments. Under the assumption that the ocean does not change its
composition, we can calculate the average time a seawater component remains in the
water, before going out as sediment. This time is called residence time.
Calculating the residence time is analogous to figuring out how long people will
stay in a museum: count the people present, and the number entering per unit time.
The ratio is the average viewing time. Similarly,
t = A/r,
(3.1)
where A is the amount present, and r is the input. Some residence times are given in
Table 3.1. Clearly, sodium and chloride have a long residence time, and silica has a
very short one. The residence time is, in essence, a measure of the geochemical
solubility (or inversely, reactivity) of the substance in question. Equation (3.1) was
once used to calculate a "salt age" for the ocean, under the assumption that the ocean
had started out fresh and retained all sodium since. This salt age came out near 100
million years. In our museum analogy, calculating the salt age corresponds to figuring
the time the exhibit opened, from the number of people present and the rate at which
they are coming in. The salt age was once useful as a minimum estimate for the scale
of geologic time.
3.4 Major Sediment Types
There are essentially three types of sediments: those that come into the ocean as
particles, are dispersed and settle onto the sea floor; those that are precipitated out of
solution directly; and those that are made by organisms. For convenience we may call
various processes, including recrystallization (crystal growth within pre-existing
solids), can be reconstructed by studying the distribution of the elements and compounds involved, within the solids and the interstitial waters. The distribution of
certain isotopes (oxygen, carbon, strontium) is of special interest in this context,
because dissolution, migration, and reprecipitation under various conditions result in
altered ratios of the isotopes.
Of course, diagenetic processes are extremely important for hydrocarbon source
rocks, migration and reservoir rock porosities, and permeabilities (Fig. 10.2).
3.3.3 Residence Time. For steady-state conditions, output must equal input. Thus the
seawater has to rid itself of all new salts coming in, in the same proportions as they
are added. Where are the "sinks" for this material? The quantitative assessment of
sinks is a major geochemical problem. For calcium carbonate the sink is calcareous
skeletons built by organisms; for silica it is opaline skeletons. The metals presumably
leave the ocean in newly formed minerals such as authigenic clay, oxides, and sulfides, and in zeolites, as well as in alteration products resulting from reactions between hot basalt and seawater, at the ridge crest. The sulfur is precipitated in heavy
metal sulfides in anaerobic sediments near the land. Some salt leaves with the pore
waters in the sediments. Under the assumption that the ocean does not change its
composition, we can calculate the average time a seawater component remains in the
water, before going out as sediment. This time is called residence time.
Calculating the residence time is analogous to figuring out how long people will
stay in a museum: count the people present, and the number entering per unit time.
The ratio is the average viewing time. Similarly,
t = A/r,
(3.1)
where A is the amount present, and r is the input. Some residence times are given in
Table 3.1. Clearly, sodium and chloride have a long residence time, and silica has a
very short one. The residence time is, in essence, a measure of the geochemical
solubility (or inversely, reactivity) of the substance in question. Equation (3.1) was
once used to calculate a "salt age" for the ocean, under the assumption that the ocean
had started out fresh and retained all sodium since. This salt age came out near 100
million years. In our museum analogy, calculating the salt age corresponds to figuring
the time the exhibit opened, from the number of people present and the rate at which
they are coming in. The salt age was once useful as a minimum estimate for the scale
of geologic time.
3.4 Major Sediment Types
There are essentially three types of sediments: those that come into the ocean as
particles, are dispersed and settle onto the sea floor; those that are precipitated out of
solution directly; and those that are made by organisms. For convenience we may call